The present invention relates to the simultaneous dual use of radiation, e.g., visible light, received from a transmitter, for both a conventional application, e.g., illumination, combined with the additional application of receiving information over a wireless media. The present invention further relates to electronic circuits capable of receiving information-bearing transmissions from a xe2x80x9cdual usexe2x80x9d transmitter and decoding or presenting the information to a user. The present invention further relates to the presentation of this data as an audio signal. The present invention further relates to the presentation of this data as a text or graphical signal. The present invention further relates to the presentation of this data as any digital or analog information signal for another electronic circuit or electronic or electromechanical system. The present invention further relates to schemes for efficient receiving and decoding of transmitted signals designed to maximize the bandwidth or information transfer capability of the optical data channel. The present invention further relates to the construction of receivers for the detection of modulated information in transmitted light.
A communication network is a means for conveying information from one place to another. The information can be digital data, audio, video, text, graphics, data, sign language, or other forms. Establishing and maintaining communication networks is one of the oldest known activities of mankind, ranging from the shouting and drum signals of prehistory through written messages, signal flags, signal fires, smoke signals, signal mirrors, heliographs, signal lanterns, telegraphs, radios, telephones, televisions, microwave signals, linked computers and the internet. Improving communication networks, and finding new, inexpensive ways to meet the growing demand for transmission capability will continue to be a major technical focus in the future. We have developed a communication network that makes xe2x80x9cdual usexe2x80x9d of sources of radiated transmissions. One implementation of this communication network, such as a wide bandwidth intranet, uses conventional light fixtures, like fluorescent lights, as transmitters. Information is encoded in the lamp light by modulating the electric lamp current with an information signal. If the modulation is done with care, the lamp continues to emit light with no perceptible visual flicker to the human eye. However, an electronic circuit or receiver can be used to decode information in the transmitted light. The lights continue to serve their primary function as a source of illumination, while simultaneously creating a wireless optical data path for information transmission. The purpose of this invention is to disclose efficient means for creating a receiver that decodes information in the lamp light with the highest possible bandwidth or information carrying capacity over the transmitted channel. While the focus of this discussion is on visible lighting systems, it is understood that the dual use transceiver concept could be applied to any system that employs electromagnetic radiation, e.g., a RADAR set, for some purpose and which could also be simultaneously modulated to provide information transmission capability.
This invention is the first to propose establishing a transceiver system using any radiating transmitter with dual utility where the primary utility is any application, not just illumination but also possibly range finding, lane marking, or other applications, and the secondary utility is communication. This invention is the first to propose the transmission of bandlimited analog information such as audio signals by using frequency modulation, which enhances the noise immunity and available bandwidth over previous schemes while specifically avoiding sensory perceptible flicker in the transmission. It is the first to propose the efficient transmission of digital data using pulse code frequency modulation, and also the first to propose encoding digital bits in sidebands around the carrier frequency of the transmitter. It is the first to propose the use of a nonlinear detector in a dual-use network receiver to improve settling and detection time of pulse-coded data. These schemes for the transmission and reception of digital data substantially enhance the available data transmission rate in comparison to schemes in the prior art, again while elimination perceptible flicker. It is the first to disclose schemes for creating multiple data transmission channels using the same transmitter, and the first to propose a receiver in a xe2x80x9cdual-usexe2x80x9d network capable of selecting one channel from a spectrum of available choices. It is the first to propose a receiver with variable xe2x80x9clock-inxe2x80x9d or transmitter capture characteristics, allowing the tailoring of the behavior of the receiver as it locks on to different transmitters. This feature could be especially important for optimizing the receiver""s behavior in way-finding applications, and in environments with many different closely spaced transmitters, to ease the process of acquiring and holding a data channel between the transmitter and receiver.
There have been a few reports of the use of visible lighting as a carrier in electronic communication networks. The earliest reference to using lighting to send electronic information as well as to provide illumination appears to be Dachs (U.S. Pat. No. 3,900,404) disclosing an analog amplitude-modulation (AM) scheme to modulate the arc current in a fluorescent lamp, the xe2x80x9ccarrierxe2x80x9d signal, with an audio information signal. King, Zawiski and Yokoun (U.S. Pat. No. 5,550,434) disclosed an updated electronic circuit that also provides for AM modulation of the arc current with an analog signal. Smith (U.S. Pat. No. 5,657,145) teaches a method for encoding low-bandwidth digital information into the lamp light using a pulsed AM technique. The encoding technique involves chopping 100 microsecond slices of current out of the arc waveform. Nakada (Japanese Patent application 60-32443, Feb 19, 1985.) reports the use of FM modulation and a frequency shift keying (FSK) scheme to transmit digital data using visible lighting. Gray (U.S. Pat. No. 5,635,915 Jun. 3, 1997 and PCT WO90/13067, Oct. 11, 1991.) has reported a phase modulated (PM) product pricing system for supermarket shelf labels where a signal is sent from visible lighting to individual product price labels on shelves to cause the listed prices to change when desired.
Other communication schemes have been proposed that do not use the lamp light as the carrier, but instead use the lamp fixture as an antenna for transmitting conventional radio wave or microwave signals. In Uehara and Kagoshima (U.S. Pat. No. 5,424,859), for example, the inventors disclose techniques for mounting a microwave antenna on the glass surface of fluorescent and incandescent lamps. Buffaloe, Jackson, Leeb, Schlecht, and Leeb, (xe2x80x9cFiat Lux: A Fluorescent Lamp Transceiver,xe2x80x9d Applied Power Electronics Conference, Atlanta, Ga., 1997) first outlined the possibility of using pulse-code modulation to transmit data with a fluorescent lamp. In the latter reference, a three-level code shifts the arc frequency to one of three possibilities every 2 milliseconds. The result is a steady light output, on average, with no perceptible flicker. This transceiver set relies on an encoded clock embedded in the three-level, transmitted waveform to synchronize the decoding process in the receiver. We have developed a dual-use transceiver system that can transmit and receive frequency modulated transmissions carrying either analog or digital data. Our experiments confirm that frequency modulated (FM) transmissions provide the best signal recovery properties in a dual-use transceiver employing, for example, fluorescent lamps as a transmitter. We have found that, with care in the design of the transceiver set, analog signals in the audio frequency range can be used to frequency modulate fluorescent lamps without causing flicker visible to the human eye in the transmitted light. For digital data signals that could include text, graphics, or other information, we have unexpectedly and fortuitously found that a two value coding, such as Manchester encoding, also allows binary bits to be transmitted with no observable flicker regardless of the nature of the data strings. We will refer to this modulation as xe2x80x9ctwo level coding.xe2x80x9d
We have designed receivers to decode both analog and also digital waveforms from transmitted lamp light. These receivers maximize the available bandwidth of the communication channel. They also incorporate unique and novel features to ensure a variety of desirable features in the receiver. For example, the receiver can be designed to provide an indication of xe2x80x9clock-onxe2x80x9d to an available transmitter, and can be designed to provide gradual or abrupt fading of the information signal presented to a user as a source becomes available or moves away. A gradual acquisition, for example, could be invaluable in using the transceiver set for direction finding. A more abrupt localization of the transmission source may be best in a transmitter-rich environment with many different competing signal sources.
In one aspect, the transceiver includes a transmitter for transmitting coded data band limited to avoid visually perceptible flicker by varying the operating frequency of a source of radiation having a primary and secondary utility. Visually perceptible flicker would be considered xe2x80x9capplication unacceptablexe2x80x9d flicker in a dual-use transceiver set employing visible light transmitters. Other applications, e.g., a radar set dual-use transmitter, might define application unacceptable flicker in different ways, e.g., flicker that interferes with radar detection. Generally, xe2x80x9capplication unacceptablexe2x80x9d flicker occurs when variations due to the secondary utility interfere with the first, or vice-versa. The system includes a receiver for receiving the encoded radiation, decoding the coded data and delivering the decoded data signal to an output stage. It is preferred that the receiver include adjustable lock-in characteristics and have a non linear detector to optimize reception performance. The receiver may also include internal data storage in which stored data can be cued by signals from the transmitted radiation. It is also preferred that the receiver include an adjustable detector to lock on to different transmitted channels or carrier waves.
The output stage may produce an audio output, text output or a graphical output. The receiver may also include a split or multi-window graphical output or display for displaying multiple data sources or synchronized data windows, for example, for text and a translation thereof. The receiver may include a digital data output or computer interface or have a graphical output or display for presenting sign language. It is also preferred that the data is sideband encoded digital data or orthogonal block frequency code data other than tri-level coding.
In particular the present invention pertains, in part, to electronic circuits capable of receiving radiated transmissions, e.g., light from a fluorescent lamp. The circuits further include means to sense light. The circuits further include means to detect changes in the frequency of the flickering of the light, where this flickering may be invisible to the human eye. The circuits further include means to demodulate the flickering to recover a signal from the transmission source. The circuits further include means to present this signal as analog data, e.g., an audio signal. The circuits further include means to detect discrete levels in the signal, and to decode these levels to reproduce a digital data or bit stream from the transmitter.
By xe2x80x9clampxe2x80x9d, as that term is used herein, it is meant a device that produces radiated transmissions, including, but not limited to, infra-red, visible, and ultra-violet light, in response to an input electrical current which flows in the lamp. A typical example is a fluorescent lamp, although other types, such as high-intensity discharge lamps, light emitting diodes, gas and solid state lasers, particle beam emitters, cathode ray tubes, liquid crystal displays, electroluminescent panels, klystrons, and masers, are also intended. Emitters of other types of radiation, such as radio antennae for applications in RADAR sets, ultrasonic transducers and mechanical blowers (xe2x80x9cradiatingxe2x80x9d air or water, for instance), for example, are also intended.
By xe2x80x9ctransmitterxe2x80x9d, as that term is used herein, it is meant a circuit in combination with a lamp that controls the flicker frequency of the radiated output of the lamp.
By xe2x80x9creceiverxe2x80x9d, as used herein, it is meant a circuit that takes as input a radiated transmission from a transmitter and that detects and presents information in the transmission, including, but not limited to, audio, text, graphical, and raw digital data signals.
By xe2x80x9csensorxe2x80x9d, as used herein, it is meant an electrical component or sub-circuit in the receiver that is capable of observing and responding to radiated transmissions. The sensor could respond, for example, by providing an electrical signal that varies according to variations in the transmitted radiation.
By xe2x80x9camplifierxe2x80x9d, as used herein, it is meant an electrical component or sub-circuit in the receiver that produces a scaled copy of the input waveform at the output of the amplifier. The scale factor relating the input and output is called the gain of the amplifier or simply the gain. The amplifier might include automatic-gain control capability to produce an output with constant mean or peak amplitude in the face of variations in the mean or peak amplitude of the input signal.
By xe2x80x9cfilterxe2x80x9d, as used herein, it is meant an electrical component or sub-circuit in the receiver that produces an output waveform that contains a limited range or band of the frequency content of the input waveform. Typical examples include low, high, and band pass filters.
By xe2x80x9cdetectorxe2x80x9d, as used herein, it is meant a component or sub-circuit in the receiver that takes an input signal that consists of a carrier wave modulated by an information signal. The carrier wave could be modulated by any means, including frequency, amplitude, or phase modulation. The detector produces an output waveform that reproduces the information in the modulating information signal.
By xe2x80x9cdecoderxe2x80x9d, as used herein, it is meant a component or sub-circuit in the receiver that takes an input signal that contains information of interest, possibly in an encoded or encrypted form. Encryption could be employed in the transmitted data, to ensure security, and compression might be used to increase the effective data transmission rate. The decoder produces an output electrical waveform that reverses the encoding, e.g., encryption and/or compression, producing, for example, an output waveform consisting of useful binary voltage levels. By xe2x80x9coutput stagexe2x80x9d, as used herein, it is meant a component or sub-circuit in the receiver that presents information to the user of the receiver in a convenient form. For example, the output stage could incorporate an amplifier and a headset to provide an audio signal of interest to the user. As another example, the output stage could take a demodulated and decoded digital information waveform as input and produce a text or graphics display. As another example, the output stage could accept commands in the demodulated and decoded input waveform and, in response to these commands, cue the presentation of stored audio, textual, graphical, or other information from a memory, disk, or other storage component in the output stage.
In one embodiment of the invention, a transceiver system is deployed that consists of an optical transmitter and receiver. The transmitter modulates the flicker (variations in intensity) frequency of the light output of a fluorescent lamp fixture. Transmission is accomplished by modulating or varying the frequency of the alternating current in the fluorescent lamp. In one typical application, the modulating signal is an audio voice recording that has been carefully bandlimited to a frequency range of 200 Hz to 3000 Hz. This ensures that the modulating signal will not create significant harmonic components in a frequency range detectable to the human eye. Light from the lamp floods an area, e.g., a room, with illuminating light that is flickering above the human visual perception range, i.e, the lamp light appears steady. The optical receiver recovers information from the light, similar in concept to the way a radio recovers signals from radio waves. In this case, however, the receiver is part of a xe2x80x9cdual-usexe2x80x9d network, in which the light serves not one purpose, as radio waves do in the case of a radio transceiver set, but rather two purposes: illumination and information transmission.
An optical sensor is employed in the receiver. This sensor is sensitive to light in the infra-red and visible light ranges. The sensor is capable of clearly resolving the flickering of the light, even though the flickering is above the human visual perception range. Note that, if the arc frequency varies over a particular frequency range, e.g., 38 to 40 kHz, the received intensity varies from 76 to 80 kHz because the intensity of the light varies with the magnitude and not the direction of the arc current. A selective bandpass filter follows the output of the sensor. This filter ensures that detected signals in the flicker frequency range are passed, while other frequency components are rejected. An amplifier may be employed before, after, or before and after the filter to condition the detected signal level.
A phase-locked loop (PLL) circuit can be employed as a detector to demodulate the FM transmission. The output of the loop filter of a properly tuned PLL circuit will correspond to the modulating information signal at the transmitter. This signal is directly useful, requiring no decoding, and can be directly passed to the output stage that consists of an audio amplifier and headset. A user can hear the broadcast audio messages over the headset.
In another embodiment, the modulating signal is restricted to a discrete set of frequencies in sidebands around the carrier frequency, i.e., a fixed base frequency for the lamp current. Different sideband frequencies correspond to different discrete tones. The identical sensor, filter, and amplifier could be used as in the previous embodiment to construct a receiver to detect the transmissions. A new detector and decoder could be added to recover the discrete tones, and interpret them as digital bits. These bits could be used to convey text or graphics information to a display, or to send commands to an output stage capable of cueing data for presentation from a storage device integral with the receiver.
In a third embodiment, digital bits could be transmitted using pulse code modulation, in which each xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d or mark bit corresponds to a specific sequence or pulse code of transmitted frequencies. The identical sensor, filter, and amplifier could be used as in the previous embodiment to construct a receiver to detect the transmissions. A PLL circuit could be used to detect the transmitted frequency levels. If the pulse codes consist of at most two frequency levels, the PLL might be configured to operate nonlinearly, saturating between its high and low frequency loop filter voltage levels, to indicate the transmitted frequency levels. The stream of transmitted frequency levels would be decoded into bits by a decoder circuit, e.g., a finite state machine, programmed to interpret the chosen pulse code as bits. Again, among other uses, these bits could be used to convey text or graphics information to a display, or to send commands to an output stage capable of cueing data for presentation from a storage device integral with the receiver.